Method for controlling an intersystem handover

ABSTRACT

According to the invention, a reduced set of transport format combinations is signalled to a subscriber station which has established a link to a second radio communication system in a second signalling channel of said second radio communication system. The transport format indicator is then selected from the reduced set of transport format combinations and used to initiate a link transfer to the first signalling channel of a first radio communication system. A full set of transport format combinations is subsequently signalled to the subscriber station in the first signalling channel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to EuropeanApplication No. 00301815.7 filed on Mar. 6, 2000, the contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for the control of an intersystemhandover, in particular the handover between a GSM- and a UMTS mobileradio system.

2. Description of the Related Art

Reference is made to the UMTS standardization documents 3GPP: 3G TS25.212 V3.1.1, 1999-12, Multiplexing and channel coding (FDD), 3GPP: 3GTS 25.222 V3.1.1, 1999-12, Multiplexing and channel coding (TDD), and3GPP: 3G TS 25.331 V3.1.0, 2000-01, RRC Protocol Specification as thestate of the art for the UMTS mobile radio system. For descriptions ofthe mobile radio system of the second generation GSM, the book by J.Biala, “Mobile Radio and Intelligent Networks”, Vieweg Verlag, is takenas a basis for the general state of the art.

In radio communication systems, for example the European mobile radiosystem of the second generation GSM (Global System for MobileCommunications), information such as speech, image information or otherdata are transmitted by electromagnetic waves over a radio interface. Atthe radio interface, one or more connections are set up between a basestation and plural subscriber stations; the subscriber stations can be,for example, mobile stations or stationary radio stations. The radiationof the electromagnetic waves takes place at carrier frequencies whichare situated in a frequency band provided for the respective system. Forfuture radio communication systems, for example, the UMTS (UniversalMobile Telecommunication System) or other systems of the thirdgeneration, frequencies in the frequency band of about 2,000 MHz areprovided. For the third mobile radio generation UMTS, two modes areprovided, one mode being termed a FDD (Frequency Division Duplex)operation and the other mode being termed a TDD operation (Time DivisionDuplex) operation. These modes find their application in differentfrequency bands; both modes support a so-called CDMA (Code DivisionMultiple Access) subscriber separation method.

Based on a parallel existence and a desired harmonization between theradio communication systems of the second and third generation,subscriber stations which have set up a connection in a radiocommunication system are to be given the possibility of handing theconnection over to a further radio communication system, which as thecase may be supports another transmission mode. Such an intersystemhandover assumes, besides a synchronization of the subscriber stationwith the radio communication system which is to take over theconnection, the knowledge of the transport format used. Here, accordingto the referenced state of the art of base stations of the UMTS mobileradio system, a subscriber station signals, during a connection setup, aso-called set of transport format combinations TFCS (Transport FormatCombination Set). With a change of the transport format used at thetime, a so-called Transport Format Combination Identifier TFCI issubsequently signaled to the subscriber station, and states whichtransport format is used out of the set of possible transport formatcombinations.

The signaling of the set of transport format combinations TFCS wouldhave a length of up to 200 8-bit bytes, depending on the number ofpossible TFCIs in a known GSM signaling channel. This length is verydisadvantageous because of the only limited capacity available on theusable GSM signal channels.

SUMMARY OF THE INVENTION

The invention has as its object to propose a method or respectively acommunication system station which makes possible a small loading of thesignaling channel of at least one of the systems during the signaling ofan intersystem handover.

According to the invention, the set of transport format combinations isreduced, and in the reduced form is used for transmission to asubscriber station on a signaling channel, for example, a GSM mobileradio system. In order to be able to work with data of the reduced dataset, the subscriber stations and corresponding network stations suitablefor this method have a memory region with corresponding data.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 is a block circuit diagram of two neighboring radio communicationsystems, a subscriber station being situated in their overlapping radiocells.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

According to the invention, two exemplary cases of a handover can bedistinguished for a subscriber station in the region of two radiosystems. It is assumed below that the handover is from a GSM to a UMTSmobile radio system, the implementation in the reverse direction alsobeing possible in principle.

According to a first case, during a connection setup procedure in a GSMmode, a service is requested by the subscriber station which cannot bemade available by the GSM system or by a GSM transmission channel(so-called bearer). In this case, the necessity exists for aservice-based connection handover from the GSM- to the UMTS mobile radiosystem.

In this phase, only a first signaling channel SDCCH (Stand-AloneDedicated Control Channel) is set up as an independent, fixedlyallocatable control channel. The transmission capacity of the SDCCHhowever appears insufficient for the transmission of the whole of theTFCS information, which in the present example can amount to up to 2008-bit bytes. In this phase, however, no service is set up. The problemis therefore solved according to the invention in that the handover islimited to a signaling channel. in the simplest form of the method, forexample, only a transport format indicator TFCI is used for the handoverto the UMTS system over the GSM signaling channel GSM-SDCCH. Here an apriori knowledge of the transport format indicator TFCI to be used mustbe present.

After a successful handover to a UMTS signaling channel, the negotiationover the service to be used can be concluded within the UMTS system, thetransport format indicator TFCI being signaled to the subscriber stationfrom the then completely available set of transport format combinationsTFCS for the traffic channel within a so-called UMTS radio bearerconfiguration information.

In order to make greater flexibility possible, a set of UMTS signalingchannels can be defined. This enables access to the network by reducedsignaling on these signaling channels. In an exemplary set of 32 TFCI,the reduced signaling can be transmitted, for example, by a simplebitmap coding of 5 bits within a signaling channel SDCCH.

In a second exemplary case, a stabilized connection to the GSM mobileradio system already exists, and the setup of a further service isrequested which can only be made available by the UMTS system. Thestandardization of the UMTS system at the present time, Release 99,assumes solely the possibility of setting up a switched service(CS—Circuit Switched). Nevertheless this leads to an enlargedpossibility of combination.

If it is assumed, as described, that the detailed TFCI or TFCSprocessing takes place within the UMTS system, then at least acombination of a signaling channel and a CS service is possible.

The advantage of the present state of the art according to Release 99 isthat a handover of transmission channels with plural services anddifferent qualities of service can be ignored on a GSM transmissionchannel; the concept of signaling by reduced data sets or counting ofindicators can be expanded in the same manner to such procedures.Because of this, a complete flexibility in the TFCI coding within thesignaling phase can be dispensed with.

If for example 64 to 128 combination possibilities are assumed between arequired CS transmission channel and a signaling channel, the TFCIcoding can be reduced to this required volume. The selection is limitedto a predefined list of possible TFCI, which represent the respectivelypossible combinations, instead of to the complete set of possible TFCI.A coding of the combination possibilities can take place, as in the casefirst considered, by a bitmap coding of, for example, 7 bits.

Summarizing, the concept embraces a substitution of the complete TFCSsignaling by a reduced set of combinations, which is known in both thesubscriber station and the network. The reduced signaling makes possiblea first handover to a UMTS signaling and data channel, or to a UMTSsignaling channel, which is subsequently used for passing on theconnection setup or service setup, making use of the then completelyavailable flexibility of the TFCI coding.

FIG. 1 shows, as a version of the first embodiment of the embodimentexample, respective sections of two mobile radio systems RS1, RS2 as anexample of radio communication systems, e.g., a GSM and a UMTS mobileradio system. A mobile radio system typically has numerous mobileswitching centers MSC or UMSC (Mobile Switching Center or UMTS MobileSwitching Center), which belong to a switching network SSS (SwitchingSubSystem) and are networked together or produce the access to a fixednetwork, and of respectively one or more base station systems BSS (BaseStation Subsystem) connected to these mobile switching centers MSC,UMSC. A base station system BSS furthermore has at least one BSC (BaseStation Controller) or RNC (Radio Network Controller) device for theallocation of radio-technical resources, and also at least one basestation BTS (Base Transceiver Station) or NB (node B), respectivelyconnected thereto.

A base station BTS, NB can set up, over a radio interface, connectionsto subscriber stations UE (User Equipment), such as for example mobilestations or other mobile and stationary terminal devices. At least oneradio cell is formed by each base station BTS, NB. The size of the radiocell is as a rule determined by the range of a general signaling channelBCH (Beacon Channel) or BCCH (Broadcast Control Channel), which istransmitted from the base stations BTS, NB at a respectively highertransmitting power than that of the traffic channels. With sectorizationor with hierarchical cell structures, even plural radio cells can beprovided for per base station BTS, NB. The functionality of thisstructure can be transferred to other radio communication systems inwhich the invention can be used, in particular for subscriber accessnetworks with wireless subscriber connection.

The example of FIG. 1 shows a subscriber station UE, which is embodiedas a mobile station and which is situated in a coverage area, and also afirst mobile radio system RS1 which supports a UMTS standard, and inaddition a second mobile radio system RS2,which supports a GSM standard.The subscriber station UE has set up a connection to the base stationBTS, shown by way of example, of the second mobile radio system RS2.

During the connection, the subscriber station UE periodically evaluatesthe transmission conditions of the radio interface to the base stationssurrounding it, such as for example the indicated base station NB of thefirst mobile radio system RS1, in order, for example with a worsening ofthe transmission quality to the base station BTS of the second mobileradio system RS2, or with an additional setting up of a service, toinstigate a handover to the base station NB of the first mobile radiostation RS1. The same method is also used, for example, withhierarchical network structures, when a connection is handed overbetween different hierarchy planes, for example, from a micro-cell to amacro-cell, which operate in respectively different frequency bands.

For performing a handover, signal channels SIG.CH1 or SIG.CH2 are usedas independent control channels which can be fixedly allocated, as hasbeen described hereinabove in the context of the preferred signalingprocedures. In a UMTS system, the set of transport format combinationsTFCS (Transport Format Combination Set) is used for signaling ahandover. This has many of the transport format combination identifiersTFCI, which are signaled to the subscriber station. If the subscriberstation UE is connected to the GSM network RS2 at a first time, then asexplained hereinabove, the reduced set of transport format combinationsTFCS is used for initiating a handover. After the setting up of acorresponding connection to the first communication system, the completeset of transport format combinations TFCS is used.

According to a modified embodiment, in particular the signaling of thereduced information set can take place over different signalingchannels, e.g., over the message control channel BCCH already existingfor other control and signaling purposes. The broadcasting of thereduced set of transport format combinations TFCS can thus in particulartake place over the radio channel or message channel BCCH. A dedicatedtransport format combination identifier TFCI is signaled and allocated,e.g., over the signaling channel SIG.CH, to the subscriber station UE,in common with the handover command.

This handover is to be capable of being performed in future radiocommunication systems, such as for example the UMTS mobile radio system,and also between radio communication systems which support differenttransmission methods. Further scenarios of the handover between like ordifferent systems and transmission methods are conceivable.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

What is claimed is:
 1. A method of controlling an intersystem handoverbetween first and second radio communication systems having first andsecond signaling channels, respectively, comprising: signaling to asubscriber station a reduced set of transport format combinationsrelated to setting up a connection to the second radio communicationsystem, in one of a dedicated second signaling channel and a radiochannel of the second radio communication system; selecting a transportformat indicator from the reduced set of transport format combinationsfor initiation of a handover to at least one of a first signalingchannel and a data channel of the first second radio communicationsystem; and signaling a complete set of transport format combinations tothe subscriber station in the first signaling channel.
 2. A methodaccording to claim 1, further comprising initiating the handover to asignaling and/or data channel of the first radio communication system.3. A method according to claim 2, further comprising coding thetransport format indicator by bitmap coding.
 4. A method according toclaim 3, wherein the first radio communication system is a UMTS mobileradio system and the second radio communication system is a GSM mobileradio system.
 5. A method according to claim 4, wherein the secondsignaling channel of the second radio communication system is one of astand-alone dedicated control channel and a radio control channel.
 6. Acommunication system subscriber station for use in an intersystemhandover between first and second radio communication systems havingfirst and second signaling channels, respectively, comprising: a memoryto store data of a reduced set of transport format combinations relatedto setting up a connection to the second radio communication system, inone of a dedicated second signaling channel and a radio channel of thesecond radio communication system; and a processor, coupled to saidmemory, to select a transport format indicator from the reduced set oftransport format combinations for initiation of a handover to at leastone of a first signaling channel and a data channel of the first secondradio communication system; and to receive a complete set of transportformat combinations transmitted to the subscriber station via the firstsignaling channel.
 7. A communication system network station for use inan intersystem handover between first and second radio communicationsystems having first and second signaling channels, respectively,comprising: a memory to store data of a reduced set of transport formatcombinations related to setting up a connection to the second radiocommunication system, in one of a dedicated second signaling channel anda radio channel of the second radio communication system; and aprocessor, coupled to said memory, to select a transport formatindicator from the reduced set of transport format combinations forinitiation of a handover to at least one of a first signaling channeland a data channel of the first second radio communication system; andto receive transmit a complete set of transport format combinations tothe subscriber station via the first signaling channel.